Maybe this is relevant: another effect that strengthens the anti-predation function of a herd is the "confusion effect" of many similar animals milling quickly about so the predator cannot fasten on any one. This is easy to demonstrate with a computer simulation and one reason that strong sex dimorphism might not be selected in such species. So a skitteriness such that the flock flies up, moving confusingly together, may put off many a hawk that is trying to fasten on a single prey. (I have had to catch up my budgie flocks many times, and it is useless to go for a flipping flying group---we predators must be able to fasten on one to be effective.)
This doesn't explain their leaving feeders but might explain frequent up-flights and confusing movements at any sign of a possible predator--if in doubt, go up; if someone else goes, go up! Starlings do this--see the Stevenson road compost! Anne On Jan 9, 2013, at 1:05 PM, Geo Kloppel wrote: > Hi Wesley, you wrote: > >> I'd actually expect the opposite: something that's called a "selfish herd" >> effect, where the larger the group, the less likely that you'll be >> depredated because by chance alone you're far less likely to be killed by >> the small number of predators in the area if you're in a group of 100 than >> if you're in a group of 2, for example. > > Maybe, but selfish herd payoff calculations have to take account of the costs > of the behavior too: increased intra-specific competition, marginalization of > the weak, and the much greater conspicuousness of large aggregations to > predators. > > I definitely take your point about finite and depletable winter food supplies > enforcing more-or-less continuous exploration for fresh resources, but why > would this result in larger aggregations being any "twitchier" than smaller > ones? > > -Geo > -- > > Cayugabirds-L List Info: > http://www.NortheastBirding.com/CayugabirdsWELCOME > http://www.NortheastBirding.com/CayugabirdsRULES > http://www.NortheastBirding.com/CayugabirdsSubscribeConfigurationLeave.htm > > ARCHIVES: > 1) http://www.mail-archive.com/cayugabirds-l@cornell.edu/maillist.html > 2) http://www.surfbirds.com/birdingmail/Group/Cayugabirds > 3) http://birdingonthe.net/mailinglists/CAYU.html > > Please submit your observations to eBird: > http://ebird.org/content/ebird/ > > -- > -- Cayugabirds-L List Info: http://www.NortheastBirding.com/CayugabirdsWELCOME http://www.NortheastBirding.com/CayugabirdsRULES http://www.NortheastBirding.com/CayugabirdsSubscribeConfigurationLeave.htm ARCHIVES: 1) http://www.mail-archive.com/cayugabirds-l@cornell.edu/maillist.html 2) http://www.surfbirds.com/birdingmail/Group/Cayugabirds 3) http://birdingonthe.net/mailinglists/CAYU.html Please submit your observations to eBird: http://ebird.org/content/ebird/ --
